Download enzyme kinetics

Enzyme kinetics
Dr Bela Goyal
Enzyme kinetics
the quantitative measurement of the rates of
enzyme-catalyzed reactions
the systematic study of factors that affect these
rates,
Catalytic mechanism of a given enzyme
Diagnostics
Therapeutics
Chemical reactions are described using
balanced equations
Changes in free energy determine the direction
& equilibrium state of chemical reactions
ΔG0 : Standard free energy change under standard conditions:
1atm,1M,25C
ΔG0′, which defines ΔG0 at a standard state of 10−7 M protons,
pH 7.0.
Order of reaction
A+B→P
 Rate1 = k1[A]n[B]m
 The sum of the molar
ratios of the reactants
defines the kinetic order
of the reaction
 ? Pseudo first order
reaction
THE KINETICS OF ENZYME
CATALYSIS
 Enzymes Lower the Activation Energy Barrier for a
Reaction
 MULTIPLE FACTORS AFFECT THE RATES OF ENZYME
CATALYZED REACTIONS
 Temperature
 pH
 Substrate concentration
Keq Is a Ratio of Rate Constants
THE MICHAELIS MENTEN EQUATION MODEL THE
EFFECTS OF SUBSTRATE CONCENTRATION
Effect of substrate concentration on
reaction velocities
Important conclusions
 Characteristics of Km
 Relationship of velocity to enzyme concentration
 Order of reaction
 Which of the following describes a characteristic feature of an enzyme obeying MichaelisMenten kinetics?
 (A) The enzyme velocity is at 1⁄2 the maximal rate when 100% of the enzyme molecules
contain bound substrate.
 (B) The enzyme velocity is at 1⁄2 the maximal rate when 50% of the enzyme molecules
contain bound substrate.
 (C) The enzyme velocity is at its maximal rate when 50% of the enzyme molecules contain
bound substrate.
 (D) The enzyme velocity is at its maximal rate when all of the substrate molecules in solution
are bound by the enzyme.
 (E) The velocity of the reaction is independent of the concentration of enzyme.
Lineweaver-Burk plot
 Linear Form of the Michaelis-Menten Equation Is Used to
Determine Km & Vmax
Competitive inhibition
KINETIC ANALYSIS DISTINGUISHES
COMPETITIVE FROM NONCOMPETITIVE
INHIBITION
Non- competitive inhibition
Non competitive inhibition
Uncompetitive inhibition
 Methanol (CH3OH) is converted by alcohol dehydrogenases to formaldehyde (CHO),
a compound that is highly toxic in the human. Patients who have ingested toxic levels
of methanol are sometimes treated with ethanol (CH3CH2OH) to inhibit methanol
oxidation by alcohol dehydrogenase. Which of the following statements provides the
best rationale for this treatment?
 (A) Ethanol is a structural analog of methanol, and might therefore be an effective
noncompetitive inhibitor.
 (B) Ethanol is a structural analog of methanol that would be expected to compete with
methanol for its binding site on the enzyme.
 (C) Ethanol would be expected to alter the Vmax of alcohol dehydrogenase for the
oxidation of methanol to formaldehyde.
 (D) Ethanol would be expected to inhibit the enzyme by binding to the formaldehyde
binding site on the enzyme, even though it cannot bind at the substrate binding site for
methanol.
Mechanism based inhibition
Covalent
Transition state analogues
Many Drugs Are Enzyme Inhibitors
 Sulfa Drugs: competition with paminobenzoic acid (PABA),
 Methotrexate: competition with dihydrofolate for dihydrofolate reductase.
 Fluorouracil: irreversible inhibitor of thymidylate synthetase
Double displacement reactions
Sequential:
 Random
 Ordered
Ping pong/Double displacement
Sequential/single displacement
reactions
Parameters used to compare the relative activity of
different enzymes or of different preparations of the
same enzyme.
Specific activity
Turnover number
Catalytic constant, kcat
Catalytic Efficiency, kcat/Km